The invention relates to a process for culturing a microalga and for extracting a polysaccharide therefrom.
For many years, hydrophilic colloidal polysaccharides, such as agar, algin, carrageenan, furcellaran and funoran, have been extracted from marine macroalgae (seeweeds). These polysaccharides are useful as thickeners, gelling agents, emulsion stabilizers, suspending agents, emollients and demulcents and are used in large quantities in the food, cosmetics and pharmaceutical industries. Polysaccharides are also used in the oil industry as thickeners in drilling muds and in fluids used for the tertiary recovery of oil from underground strata. It is estimated that the total value of polysaccharides produced from seaweeds in the United States in currently about two hundred million dollars per year.
However, the recovery of polysaccharides from macroalgae is attended by numerous difficulties. It is not practical to artificially culture macroalgae on the scale necessary for large-scale polysaccharide production, due to their large size and the resultant space required. Accordingly, the macroalgae must be harvested from their natural sites in shallow water near to sea coasts. Many of the coasts on which the macroalgae occur are rocky and are subject to severe storms at certain times of the year. The growth of the macroalgae in various parts of the United States is hindered by over-harvesting, coastal water pollution, sea urchin infestation and other factors. Moreover, the labor involved in harvesting natural macroalgae is difficult arduous and expensive. Furthermore, the macrolagae may be contaminated by large amounts of foreign matter, such as sand, and require considerable pretreatment to remove such foreign matter before the polysaccharide is extracted from the macroalgae. It is difficult to produce a consistent product from macroalgae and it is necessary to monitor very closely the properties of the polysaccharide, and often to blend polysaccharide from different batches of seaweed, in order to ensure that the thickening properties of the polysaccharide remain constant, since such properties vary not only with the type of seaweed and the site on which it grows, but also with the time of the year. Finally, a considerable portion of the world's macroalgae resources are located in countries of dubious political stability, so that the international trade in macroalgae polysaccharides is vunerable to interruptions for political reasons.
In view of the difficulties associated with the production of polysaccharides from macroalgae, attempts have recently been made to extract such polysaccharides from microalgae, several of which are known to exude polysaccharides into the medium surrounding them at various stages during their life cycle; see, for example, Percival and Foyle, Extracellular Polysaccharides of Porphyridium cruentum and Porphyridium aerugineum, Carbohydrate Research 72, 165-176 (1979). U.S. Pat. No. 4,087,936, issued May 9, 1978 to J. G. Savins and M. L. Anderson, describes a process for the extraction of a polysaccharide from P. cruentum. This process is carried out in fermentation vessels using either artificial light or sunlight and thus the process can be much better controlled than can a process using marine macroalgae. Such processes using microalgae under closely controlled conditions can be expected to yield a much more uniform product than is usually obtained from marine macroalgae.
Unfortunately, the yields of polysaccharide from microalgae cultures have hitherto been disappointing. For example, Medcalf et al., Carbohydrate Research 44, 87-96 (1975) report a yield of only 0.47 g. of polysaccharide per liter of culture medium containing P. cruentum. Such low yields would render the resultant polysaccharide uncompetitive with comparable polysaccharides isolated from marine macroalgae.
We have now concluded that one of the reasons for the previously-reported low yields of polysaccharides from microalgae is that previous processes have concentrated their efforts on maximizing the amount of extracellular polysaccharide produced and have been concerned mainly with the extraction of the extracellular polysaccharide; for example, in the aforementioned U.S. Pat. No. 4,087,936, the culture medium containing the microalgae is centrifuged to remove the cells therefrom and only the cell-free growth medium is used.